The present invention generally relates to apparatus and methods to measure the rate of penetration (ROP) of a bottom hole assembly (BHA) into a subterranean formation. More particularly, the present invention relates to measuring the rate of penetration of a bottom hole assembly into a subterranean formation using accelerometers. More particularly still, the present invention relates to accurately measuring the rate of penetration with accelerometers using an advanced calibration and zeroing apparatus and method.
Boreholes are frequently drilled into the Earth's formation to recover deposits of hydrocarbons and other desirable materials trapped beneath the Earth's crust. Traditionally, a well is drilled using a drill bit attached to the lower end of what is known in the art as a drillstring. The drillstring is a long string of sections of drill pipe that are connected together end-to-end through rotary threaded pipe connections. The drillstring is rotated by a drilling rig at the surface thereby rotating the attached drill bit. The weight of the drillstring typically provides all the force necessary to drive the drill bit deeper, but weight may be added (or taken up) at the surface, if necessary. Drilling fluid, or mud, is typically pumped down through the bore of the drillstring and exits through ports at the drill bit. The drilling fluid acts both lubricate and cool the drill bit as well as to carry cuttings back to the surface. Typically, drilling mud is pumped from the surface to the drill bit through the bore of the drillstring, and is allowed to return with the cuttings through the annulus formed between the drillstring and the drilled borehole wall. At the surface, the drilling fluid is filtered to remove the cuttings and is often recycled.
In typical drilling operations, a drilling rig and rotary table are used to rotate a drillstring to drill a borehole through the subterranean formations that may contain oil and gas deposits. At downhole end of the drillstring is a collection of drilling tools and measurement devices commonly known as a Bottom Hole Assembly (BHA). Typically, the BHA includes the drill bit, any directional or formation measurement tools, deviated drilling mechanisms, mud motors, and weight collars that are used in the drilling operation. A measurement while drilling (MWD) or logging while drilling (LWD) collar is often positioned just above the drill bit to take measurements relating to the properties of the formation as borehole is being drilled. Measurements recorded from MWD and LWD systems may be transmitted to the surface in real-time using a variety of methods known to those skilled in the art. Once received, these measurements will enable those at the surface to make decisions concerning the drilling operation. For the purposes of this application, the term MWD is used to refer either to an MWD (sometimes called a directional) system or an LWD (sometimes called a formation evaluation) system. Those having ordinary skill in the art will realize that there are differences between these two types of systems, but the differences are not germane to the embodiments of the invention.
An increasingly popular form of drilling is called “directional drilling.” Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction. Directional drilling is advantageous offshore because it enables several wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which increases the production rate from the well.
When drilling subterranean wellbores, it is often desirable for the operator to know the rate of penetration (ROP) of the drillstring into the formation for any particular instance. If the measurement is taken at the drill bit, ROP can be a direct measure of how much progress the drilling apparatus is making in a particular formation. As there is much variability among subterranean formations, the rate of penetration for a particular drilling apparatus is expected to change over time.
In addition to its primary use as a measure of success in drilling, operators may also use ROP to determine changes in the formation, wear on the drilling apparatus, and data collection triggering for MWD tools. An accurate, at-the-bit, time-delimited measurement of ROP can help operators identify formation transitions. For example, if ROP is measured at 30 inches per hour at one depth and 40 inches per hour at another depth, operators can use that change In ROP to estimate a change in relative hardness of the formation between the two recorded depths. Furthermore, if ROP measurements gradually (or suddenly) drop as a wellbore is drilled, operators at the surface can use the data received to determine whether the drill bit has become substantially worn, necessitating replacement.
Finally, an accurate measure of ROP is advantageous for MWD operations as well. Most MWD operations require the collection (and storage) of large amounts of data. Often this data would be too voluminous if transmitted continuously, therefore sampling at time-delimited intervals is typically employed. With an accurate measure of ROP, an MWD operator can set the data acquisition interval to maximize the benefit of the measurements. If the ROP is slow, data measurements taken at short intervals waste telemetry bandwidth. In contrast, measurements taken too infrequently would not yield a complete data set. Therefore, the use of an accurate ROP measurement enables optimized MWD operations that get the most utility from a limited telemetry bandwidth.
Because ROP is typically reported in feet per hour, it is often difficult to estimate actual ROP at the drill bit from the surface. Traditionally, ROP measurements were made at specified intervals by measuring the amount of drill pipe paid out at the surface over said intervals. Because a typical drillstring can be several thousand feet long, ROP measurements made at the surface rarely correlate to the actual rate of penetration experienced by the drill bit. Drillstrings over several thousand feet in length act as elastic members and can stretch and hang-up at various points along their length, making surface ROP measurements estimates, at best.
One method that has been employed to determine at-the-bit ROP has been through the use of accelerometers. Accelerometers have been used, with limited success, to determine the acceleration along the axis of the drill bit downhole. This acceleration data is then integrated to yield a velocity along the axis of the drill bit. The accuracy of these types of measurements leave much to be desired. Primarily, during drilling, the bit assembly undergoes significant vibrations and other associated movements as the formation is cut. Furthermore, with directional drilling technology being quite advanced, the component of gravity will have a different effect on the error component of the accelerometer as the drillstring is drilled further into the formation. For this reason, the error component of the accelerometer in the bit axis will change over time. Furthermore, the process is made even more difficult by the relatively low velocities (on the order of inches per hour) that are to be detected by the accelerometer. Left unchecked, the undesired components experienced by the bit axis accelerometer can dominate the reading, leaving little chance for an accurate ROP extrapolation. A more accurate at-the-bit ROP measurement apparatus and method would be highly desirable.